Temperature compensated wire strain gage



% A ril 5, 1966 J. D. RUSSELL TEMPERATURE COMPENSATED WIRE STRAIN GAGE 5Sheets-Sheet 1 Filed June 15, 1960 INVENTOR John D. Russell April 5,1966 J. D. RUSSELL 3,245,016

TEMPERATURE COMPENSATED WIRE STRAIN GAGE Filed June 15. 1960 3Sheets-Sheet 2 /54 8 7 I60 I38 9 /62 /44 /5a 36 INVENTOR John D. RussellFig. n. Fig. l2.

ATTO RNEY TEMPERATURE COMPENSATED WIRE STRAIN GAGE Filed June 15, 1960 3Sheets-Sheet :5

(Illlllll' 'IIIIIIIII /20 me O I |J= Fig l8 20a\ /98 T 200 /9a /20 /20Fig. l9. T 194 I f 220 INVENTOR John D. Russell RNEY United StatesPatent 3,245,016 TEMPERATURE COMPENSATED WIRE STRAIN GAGE John D.Russell, Malibu, Calif., assignor to Microdot Inc, South Pasadena,Calif., a corporation of California Filed June 15, 1960, Ser. No. 36,3043 Claims. (Cl. 338-2) This application is a continuation-impart of US.application Ser. No. 754,956, filed August 14, 1958.

The present invention relates to improvements in inherently compensatedstrain gages and to methods for producing the same. More particularly,the invention relates to temperature compensated weldable strain gages;gages which when mounted on a test specimen which is free to expand,will be substantially insensitive to changes in temperature, thus havinga very low or zero resistance change with such variations intemperature. The invention further relates to methods for preparing theimproved and herently compensated wire-strain gages described herein.

Resistance wire-type strain gages as originally known were characterizedby undue sensitivity to temperature variations. In early research andstudy with these gages the temperature variations were not consideredunduly significant for many purposes and thus while temperaturecompensation was considered desirable, it was not immediately consideredcritical. However, as the possible areas of utility of such gages weremore fully explored, the realization of the difficulties inherent inadapting these initially devised gages for new uses was had. Too, inmore recent years an increased demand for gages providing compensationfor temperature changes has become necessary by virtue of the testingand development of devices in the rocketry and missile fields, whererapid variations of hundreds or even thousands of degrees areencountered.

The need for temperature compensation of strain gages also arises fromphysical factors inherent in the gage itself. Thus, the resistance ofmost materials used in such gages varies with temperature; and a secondtemperature variable is introduced where the thermal coefi'icient ofexpansion of the strain gage wire is different from that of thestructure to which it is bonded.

A form of temperature compensation has been attained heretofore incertain instances by installation of a second strain gage, often knownas a dummy gage, on an unstressed piece of the same metal as that towhich the active strain gage is bonded. If the two pieces of metal aresubjected to the same temperatures during testing, compensation isprovided for thermal resistance changes in both the test and the dummygages, for some purposes, even though the active and dummy gages are notinherently compensated. This will be true whether resistance changesoccur due to the thermal coefficient of the wire in the gage or to thedifferential expansion existing between the gages and the metal to whichthey are bonded. It is noted that this will be true even though bothgages have a relatively high sensitivity to temperature, so long as theyhave the same sensitivity, and, as noted above, are subjected toidentical temperatures since the overall effect is cancelled out in thebridge circuit.

Thus, it will be evident that the dummy gage technique does not providea temperature compensated gage, but rather a temperature compensatedsystem and bridge circuit composed of two or more matched uncompensatedgages.

However, merely employing an extra, or dummy, gage in an adjacent leg ofa Wheatstone-bridge circuit or modification thereof, in addition tobeing cumbersome is not enough to ensure complete or even adequatetemperature compensation under the increasingly severe test conditionsnow extant. Thus, full temperature compensation requires that bothstrain gages, the active and the dummy be attached to the samestructure, and be located as nearly adjacent as possible. Nor will thisprecaution sufiice even in the most standard operating conditions if thetwo strain gages do not have identical thermal coefficients ofexpansion, resistance change and the same gage factor; the gage factorbeing the dimensionless relationship between change in gage resistanceand change in length, or strain. Gage factor is, specifically, themeasure of the amount of resistance change for a given strain and isthus an index of the strain sensitivity of the gage.

In any event, the dummy gage method of temperature compensation isinadequate and ineffective where rapid and wide variations intemperature are encountered; for the test or active gage and dummy gagein such environments cannot be located in such a manner as to guaranteethat the temperatures of the two gages, dummy and test, will be the sameunder mounted test conditions. Illustratively, if the strain on the skinof a missile is to be measured during re-entry into the atmosphere, thedummy gage cannot be placed directly on the skin because it will sensestrain introduced by stress. Further, if the dummy gage were placed inadjacent parallel relation to the test or active gage, it would cancelthe reading of the active gage, assuming the latter gage is connected toa bridge circuit, to cancel the temperature effects referred to above.Thus, any high stress or strain in the metal of the missile surface orskin, for which purpose the active gage is applied thereto, would, ofnecessity be undetected. Placing the dummy gage at right angles to theactive strain gage on the missile surface is also ineffective, sinceunlike a simple tension link, the stresses exerted are complex, and thecombined strain in the two divergent directions of stress would induce asingle signal of meaningless import, it being unknown as to which of thegages provided which proportion of the total signal transmitted.

It should also be noted that even if the dummy gage could be placed onthe missile surface in close physical proximity to the active gage,there would still result significant differences in temperature betweenthe two gages induced by rapid temperature changes and/ or variabledissipation caused by underlying structural members. Nor is it practicalto attach a dummy gage to a separate unstressed plate of the same metalas that of the test strain gage in the same bridge circuit sinceattachment to the exterior skin or surface of the missile without makingthe dummy gage an integral part thereof would result in its subjectionto high-velocity passage through the atmosphere and resultingtemperature variations widely divergent from those of the test gage.Similarly and obviously no benefit is to be derived from locating adummy gage within the missile while the test gage is attached to themissile skin.

The above references are illustrative of the existing problems in thesefields for which the dummy gage technique is unable to provide thenecessary solutions. It will be obvious to those skilled in the art thatthe problem of temperature compensation under test conditions andrelated problems as well, also exist in many other areas, such as forexample, in the strain measurement of metals employed in jet propelledaircraft which are capable of such rapid acceleration and high speedscausing rapid and significant temperature and pressure variations aswell as sustained elevated levels thereof in the metal of the planesurface due to air friction and the like; in the construction of jet androcket engines; and indeed normally wherever substantial and rapidtemperature gradients occur.

Hence, in view of the ever increasing severity of test conditions andthe inability of the dummy gage technique to provide temperaturecompensation operative under these conditions, the attainment of anaccurate and efficient inherently compensated Wire strain gage hasevolved into a most significant endeavor.

It should be noted that even in standard, less severe environments wherethe dummy gage technique referred to above can be employed to betteradvantage, its effectiveness as an accurate measure of strain ismarkedly enhanced by substitution therein of inherently compensatedgages which provide less scatter in temperature sensitivity than gagesselected merely as identical in structure and which possess a lowtemperature sensitivity unlike identically constructed test and dummygages each of which manifests an elevated temperature sensitivity.

While the inherently compensated gage can always provide, it isobserved, an improvement in the accuracy of a dummy gage system,employment of the dummy gage system known heretofore will not improvethe accuracy of the measure derived from use of aninherently compensatedwire strain gage. indeed, where the dummy gage and active gagetemperatures are widely divergent, a cir cu-mstance which can so readilyoccur in the increasingly severe conditions of test and developmentstudies, the added number of variables resulting would succeed only inintroducing error into the measurements obtained by virtue of such illadvised usage. I

inherently temperature compensating Wire strain gages, often referred toas self-ternperature compensating, have been known heretofore. Thesehave been of the bonded variety wherein the resistance wire is bonded bymeans of cement, glue, cellulose, Bakelite, resin or a similar adherentmaterial to the surface of a sheet of insulating material such as paper.The paper insulating support is, in turn, bonded to the test specimen inwhich strain is to be measured. In these gages the fine kire resistancefilament is disposed as a grid and composed of two different materials;for example, a copper-nickel alloy and nickel connected in series. Thelengths of the two different wire materials are proportioned so that thetotal increase in resistance with temperature, due to both thecoefficients of resistivity and thermal expansion, of one material, e.g. copper-nickel alloy, is to a large extent cancelled by a decrease inthe resistance of the other material, e.g. nickel. Since the temperaturecoefficient of expansion varies with different materials, it is, ofcourse, necessary to select the proper gage for the material beingtested. Gages of this type are presently used on materials having thetemperature coefficients of expansion of annealed mild steel and 24ST6duralumin um. lllustratively, a difference in coeliicients of expansionof IX per degree Fahrenheit (per deg. F.) results in a gage indicationof 1 micro-inch per inch per deg. F. One difficulty inherent in thesegages has been their limitation for use within comparatively narrowtemperature ranges in terms of the testconditions where stress gages areused or needed. Thus, these gages are excluded from use for the majorityof demanding test pro cedures employed in the fields of aerodynamic,jet, missile and rocketry research and development. lllustrative'of thetemperature ranges within which these compensating strain gages knownheretofore are purported to have some value are +50 F. to +250 F. andfrom 50 F. to +300 F. From curves purporting to further characterizethese gages it appears that the margins of error embraced within thenarrower range may be as much as 50 micro-inches per inch while for thebroader range recited the error may extend to in excess of 300micro-inches per inch. Temperature use limitations are also introducedby the nature of the bonding material employed. A still furtherditiiculty with this type of gage is that once manufactured the gagecannot be readily or conveniently adjusted; nor can it be given anaccurate unmounted sensitivity calibration; nor indeed can a check ontemperature compensation or temperature sensitivity be effected aftermounting thereof on a specimen to be tested, in most in stances. Thus,there is no accurate correlation between unmounted and mountedsensitivities.

It will, for example, be evident that temperature compensation of astrain gage cannot be accurately determined after it is mounted on acomplex test specimen where the degree of restraint with temperaturechange is a complex and normally indeterminable factor. Thus, if thetest specimen is temperature calibrated after gage installation the gageresistance change may be due to one or a plurality of strains; eig. tostrains induced by free expansion or strains caused by thermally inducedstresses; with no means for determining which proportion of the measuredsignal is caused by one or another of these strains.

Thus, in accordance with the present invention there is provided aninherently temperature compensated wire filament weldable strain gagewherein temperature compensation and calibration have been. reliably andaccurately introduced prior to mounting of the gage on a metal piece ortest specimen; thus effecting the preparation of a gage which moreefficiently measures strains occasioned by stresses introduced into testspecimens whether these stresses are induced by thermal elfects, aswhere a metallic test piece is restrained, in which case the gage isdesirably sensitive to temperature modification; where such changescause a stress on the piece orstructure tested; or where these stresseson the test specimen are introduced by pressure, force, acceleration orthe like.

To accomplish these and other objectives of the present invention thereis provided a wire strain gage which, when mounted on a test specimenwhich is free to expand, will have a very low or zero resistance changewith changes in temperatures; in effect, a gage made insensitive tostrain or elongation; a state attained only in a gagewherein the metalexpands freely with increasing temperature.

Such strains, referred to as free thermal expansion strains are not,however, the only factors required to be eliminated or cancelled inorder to provide a meaningful temperature compensated gage. Other andinterrelated factors upon which temperature compensation depends are thethermal coefficient of resistivity and the thermal coefiicient ofexpansion of the wire. 0

These factors, noted to illustrate the selectivity and delicacy of theproblems inhering in formulating a gage of dependable mountedsensitivity prior to mounting, can, however, be counteracted by alteringthe thermal coelficient of resistivity or the thermal coefficient ofexpansion of the wire filament of the gage. Attempts to provide acompensated gage by balancing of these factorshas been attemptedheretofore. However, a practicable basis for effecting this onindividual gages has not been known hitherto. Thus, the known proceduresemploy a batch or spot system of selection. This practice involvesselection of a spool of wire sufiicient to form a large number of gageswith a supposed correct coeflicient of resistivity, from which a batchof gages theoretically compensated for a metal with a particularcoefiicient of expansion is then formed. This procedure has been found,however, to introduce a very large and significant amount of error, forthere exists a surprisingly intolerable scatter or variation in theactual sensitivities of the gages made from even the most uniformlyconstituted spool or similar length of wire. Thus, the differences insensitivity between two or more gages so constructed and prior tomounting thereof on a test specimen have been found to be as much as1000 micro-inches per inch of apparent strain for a temperature changeof 500 F. This would correspond to an erroneous stress measurement ofapproximately 30,000 p.s.i. in steel. Thus, these hatch systems knownhereto fore when employed with the bonded gages, prepared as describedabove, result in compensating gages with an intolerable scatter orvariation over any temperature range akin to that which they wouldexperience in modern test conditions. A further disadvantage'resides inthe fact that these bonded gages cannot be readjusted for another testmaterial once they have been prepared, even were no scatter inherenttherein. This is most commonly due to the method of bonding of the wirefilament to its support and the bonding of the support itself to thetest specimen. Thus the adherent or bonding agent has only a limitedtolerance for variations in temperature before attaining a final setwhich cannot be disturbed without destroying the usefulness of the gageand indeed the gage itself.

The present invention provides, therefore, a weldable resistance wirefilament strain-gage duly and accurately calibrated and compensated fortemperature prior to its mounting for use on a test specimen; a gagewhich, in addition, can be readjusted in its temperature compensa .tionrepeatedly should it be decided to use a particular gage or pluralitythereof with test material other than that for which it was originallyprepared. Further, it is feasible in accordance with the presentinvention to check on the sensitivity of each individual gage preparedprior to removal thereof from the calibration and adjustment device andthus, of course, prior to mounting thereof in a test procedure. Stillfurther, the adjustment, readjustment and calibration of individualgages is accomplished, in accordance herewith, in a manner both rapidand economically practicable.

It will be apparent, however, that it is also feasible, althoughunnecessary, to employ a batch system in the preparation of thetemperature compensated strain gages of the instant invention whereby aspool of wire having an announced thermal coefficient of resistivity mayhave imparted thereto an average sensitivity irrespective of theintolerable scatter which occurs, so that the fine adjustment insensitivity required by the individual gage subsequently will be held toa minimum, even though adjustment can be made over a wide range. In thismanner too, some of the gages of the batch may even attain the desiredsensitivity. The remaining gages of the batch or group can then beindividually adjusted; a procedure which could not be performed with thetemperature compensated gages employed heretofore.

In general, the gages calibrated and compensated for temperature andused in the practice of the present invention are the weldable straingages described in copending United States patent application Serial No.754,956, filed August 14, 195 8, by the inventor herein and of which thepresent application is a continuation-in-part. These gages are adaptedfor coupling or attachment to the test specimen without the use oftemperature sensitive adherents. These gages have the further advantageof being well adapted for use under extreme temperature conditions aswell as for use at ambient temperatures. Further, the gages of theinvention are well suited for use with test specimens having curved orirregular surfaces and are, in addition, attachable to the test specimenin such a manner as to effect a strain responsive coupling over theentire length of the resistance filament contained within the gage, andmost significantly, without the use of cements or other sensitivebonding materials. Thus, these strain gages can be welded to the testspecimen, either by application of normal arc welding techniques or bymeans of sonic welding.

In generalized form, the gages employed herein comprise a resistancewire or strain-responsive filament carried by a supporting element, suchas an external housing of metal, and mechanically locked to thesupporting element by means of a dense mass of insulating materials insuch fashion that longitudinal changes in the dimension of thesupporting element caused by deformation of a test specimen to which itis attached are imparted to the resistance wire or filament via the massof insulating material. Thus, for example, the insulating material maybe a compactible or compressible solid possessing both thermal andelectrical insulating properties, which is deposited around the filamentand enclosed within a metallic tube permanently deformed, as by drawing,crimping, etc., to exert a compressive force on the insulating material,

thereby forcing it into firm compressive contact against the resistanceelement and effectively frictionally coupling the element to theexternal tube. Accordingly, with the tube attached along its entirelength to a test specimen, strain applied to the tube from the testspecimen is transmitted through the insulating material and hence to theresistance element contained therein.

The external housing of the gage is formed of a freelyyielding ordeformable material such, for example, as thin sheet steel, stainlesssteel, platinum, aluminum, tungsten, tungsten alloys, etc., which isreadily adaptable for rigid bonding, as by welding, to a metallic testspecimen. In this manner, the gage can be securely fastened to a testspecimen very quickly without the necessity of waiting for a bond to setor cure as in the conventional types of bonded gages referred tohereinabove. The unique structural characteristics of the gage permitits being mounted by welding over its entire length to either a flat orcurved surface and without danger of damaging or shorting in theinternal resistance wire or filament which is electrically insulatedfrom the housing by the surrounding mass of insulating material, andalso render the gage adaptable for high temperature measurement work dueto the thermal insulating properties of the assembly, as Well asmeasurement work performed under ordinary or ambient conditions oftemperature.

In order that the invention may be readily understood reference is hadto the accompanying drawings, forming part of this specification, andillustrating, by way of example, certain apparatus embodying theinvention and by which the method of the invention is carried out. Inthese drawings:

FIGURE 1 shows in a view which is partly elevational and partlyschematic the gage sensitivity adjusting and measuring system of thepresent invention;

FIGURE 2 is a longitudinal sectional view of a typical strain gageemployed in the practice of the invention and shown in operativeposition with respect to a test specimen;

FIGURE 3 is a sectional view taken along line 33 of FIGURE 2,illustrating at the same time the preferred modification of the presentinvention;

FIGURE 4 is an elevati-onal view of a modified form of resistanceelement useful in the device of FIGURE 2 as well as in other gagestructures of the class employed herein.

FIGURES 5 and 6 are transverse sectional views illustrating, incross-section, two modified forms of strain gage embodying theprinciples of the gages for use in the present invention;

FIGURE 7 is a longitudinal sectional view illustrating another form ofstrain gage for use in the present invention, and the manner in whichthe same is attached to a test specimen;

FIGURES 8 and 9 are transverse sectional views taken along lines 8% and99, respectively, of FIGURE 7;

FIGURE 10 is a transverse sectional view illustrating the manner inwhich a strain gage of the type illustrated in FIGURE 2 can be welded toa metallic test specimen;

FIGURE 11 is a view, partly in top plan and partly in horizontalsection, of a further embodiment of a strain gage which can be employedin the practice of the present mvention;

FIGURE 12 is a transverse sectional view taken along line 12-12 ofFIGURE 11;

FIGURE 13 is a fragmentary view, in longitudinal section, representing amodified form of strain gage for use in the present invention;

FIGURE 14 is a plan view illustrating the manner of constructing anotherform of strain gage of the class employed herein;

FIGURE 15 is a transverse sectional view taken along lines 15-15 ofFIGURE 14 illustrating the completed strain gage of FIGURE 14;

FIGURES 16 to 19 inclusive are elevational views illustrating steps inthe manufacture of another form of strain gage embodying the basicprinciples of the class of gage having utility in the present invention;

FlGURE 2 is a transverse sectional view taken along line -20 2 0 oiFIGURE 19;

FIGURE 21 is an clevational View of ah electroplating ,ji'g,illustrating a relatively simplified method for massproducing resistancefilament-lead wire units useful in the strain gages of the invention.

Referring with greater particularity to the drawings, FIGURE 1illustrates means for effecting calibration and adjustment oftemperature compensated wire strain gages, and comprises the circuitarrangement and. apparatus employed for this purpose. The system shownmay be viewed for convenience ascomposed of four compohents whichcomprise an oven-immersion or dunking means'indicated in its entirety bythe numeral 1; an oven heating means indicated broadly by the numeral 2;a gage clamping or engaging means 3 and a gage sensitivity measuringassembly 4.

J Treating or": these component assemblies and their interrelationpredominantly in sequence, the oven-immersion device 1 comprises asolenoid actuated air-valve 5 which is energized by closure of theswitch 6 connected to a current source by means of the cord and plug 8;and which is provided with an air inlet 1t) and an air vacuum outlet 12.When the switch 6 is closed the solenoid air valve 5 is opened to admitair which is conducted thereto from an air supply not shown) through theinlet 10. The open valve means 5 passes the air through the air supplyduct 14'to the long stroke air cylinder 16 which provides pressure topush the piston rod 18 downward; one end of said rod 18 being disposedin slideable engagement within said cylinder. During this operation line17 is open through the air duct 12 to the atmosphere. The rod 1 isattached at its alternate end to the carriage assembly 20, and in thedownward movement of the rod 13 impelled by the air pressure within thecylinder 16, the carriage 2d isforced downwardly also on the guideways'22 with which the carriage 20 is in slideablc engagement. In thismanner the clamping means 3 attached to the carriage 20 is also moveddownwardiy and into the oven 26; the cover 28 of said oven 26 beingmoved by suitable means to an openposiion, simultaneously with thedownward movement of the rod 18 the carriage 2i) and the clamping means3. The cover 23 is mounted on the oven 26 in such a manner as to slidein a horizontal plane in opening and closing of the gage immersion port'30 (indicated by a dotted line) through which the clamping means 3passes to and from the oven 26. The heating element 46 and oven 26 aswell as the gage clamping means 3 are formed of a material which isnonreactive at the elevated temperatures which the oven 26 is capable ofattaining, i.e. 2090 F. The temperature of the oven 26 is maintained atthe desired level by suitable and con: ventional means which may includea monitoring thermocouple (not shown) disposed within the aforesaid oven26.

A plurality of ovens or a single oven with a plurality of partitions maybe and is most desirably employed for heating of the gage filament. Inthis procedure the individual ovens or partitions are erected tofunctionat three different temperature ranges. Each oven or partitionmay, in turn, be supplied with an oven immersion system so that the gageto be adjusted may "be removed from one system to another as it becomesdesirable to use a particular oven or partition having a particulartemperature range. Alternatively, a single immersion assembly may beemployed, which can be moved from oven to oven, or partition topartition or theappropriate oven or partition moved under the assembly.By this arrangement, there would be available, illustratively, a heatingunit, -e.g. an oven or partition of an even, having a temperature ofapproximately 600 F. to be used for checking the temperature sensitivityof a gage as will be discussed hereinafter. In this instance the gagecould be immersed inthe heating unit from room temperature to 600 F. Asecend heating unit would be set at a temperature of 850 F. to be usedfor immersion of a gage therein when a lowering of temperaturesensitivity is desired 'as will be elaborated upon in detailsubsequently. A third oven on the other hand, would be set at 1100 F. orhigher, as also described elsewhere herein, and be used to heat the gageand gage filament to elevate the temperature sensitivity thereof.

Opening of the switch 6 results in the passage of air pressure throughair ducts 1d and 17 to the lower end of the air cylinder '16, while airlines or ducts l2 and "14 are vented to the atmosphere; thus exerting"an upward pressure and lifting action on the head of the 'rod 18. As aresult the piston rod rises in the cylinder 16 lifting the carriage 21KBand the clamping means 3 attached thereto. In this manner that portionof the clamping means 3 remote from the carriage 2i), in which the gage43 "is *rem'ovably mounted is withdrawn from the oven 2-6. As theclamping means clears the portpr orifice 3i) in its upward passage thecover member Z8 of the oven 26 is returned to its closed position overthe orifice 30 and the switch 32, if desired, is returned to its openposition, thus de-energizing the resistance element or a plurality ofresistance elements illustrated at do in theov'en 2d.

The clamping or gage mounting means 3 provides'for insertion of the gage48 'in position in the bridge circuit of the gage sensitivity measuringassembly to be 'described in detail hereinafter ahd wherein the gage 48to be adjusted constitutes the resistance R The clamping means 3comprises a solenoid 50 which is energized by closure of the switch52'c'onnected to a current source by means of the cord and plugid. Whenso energized, air pressure, illustra'tiv'ely in the "order of onehundred pounds per square inch (p.si.) is passed from an air supply (notshown) by means 'of the air inlet 55 through the solenoid air valve Shand the airline 56 to the air cylinder 53. When de-ene'rgi-zed byopening of the switch $52, the air valve inlet 5% is closed and theunre'newed'air in the cylinder '58 is simply bled oil. through the airduct (iii and the solenoid air valve Stl into thc'air outlet "62. Theair cylinder 5'8 when receiving air under pressure from the solenoid 5h,forces the piston rod 64 'slidea'bly mounted therein, downwardly, thusforcing the spring-biased cross-member 66 atfixed to the opposite andlower end of the air cylihder'SS downwardly as well. The cross-member ascompresses the spring elements 68 positioned between the aforesaidcross-member 66 and the anchor bar Hi. The latter member 70 and the aircylinder 58 are mounted in fixed relation to the carriage 20 'of theoven immersion system It. The air cylinder 58 is ai'hxed to the carriage2G by means-of the bracket member 72. Tubes 74 are 'affixed at theirupper'ends to the anchor bar 70 and are positioned in spacedsubstantially parallel relation to each other. They are, in addition,suspended downwardly from the anchor bar 70 and have an insulatedcross-brace 75" provided across their lower free ends. The orifices of'the tubes 74 are continuous Withorihces '76 defined in the anchor bar70; each tubular orifice and its anchor bar o'riiice extension '76presents a passage for each of the wire members 78 which are attached tothe under surface of the aforesaid cross-member as, on the side oppositethe attachment thereof to the piston rod ti-t; the upper terminal-end'oieach of said wires 78 being aihxed to said cross-member 66 at a positiondirectly above each of the orifices '76. Attached to' the terminal endsof these wire members 78 which are extended beyond the free lowerends ofthe tubes 7 -5 are the expanded pads or clamps 3%. These latter members8%) cooperate with the 'pe'riphe'ryof the free end of each or theaforesaid tubes '74 to removably engage and hold the opposite ends of astrain gage'dd to headjusted'according to the practice describedherein'when the switch is open and the spring-biased cross-member 66 andthepiston rod o sare at rest in anelevated position. Thus, energizingthe solenoid air valve 50 and air cylinder Sriby the closure of theswitch 52 effects lowering of the rod 64, the cross-member 66 andconsequently the wire clamp supporting wires 78 and clamps 80 to causerelease of the gage 48 ends from between the clamps 80 and thecooperating free ends of the tubes 74.

When mounted in the clamped position as described, the gage 48constitutes the resistance R of the bridge circuit of the gagesensitivity measuring system 4. The remaining portion of the bridgecircuit and measuring system 4 is composed broadly of an SR-4 (a tradename) strain indicator 82, manufactured by the Baldwin-Lima- HamiltonCorporation; a dummy gage resistance element 84; the conductors 86 and88 to the opposite terminals of the gage 48 undergoing adjustment whenthe clamping system 3 and gage 48 are lowered into the heated oven 26;and the compensating lead wire 90 a portion of which also enters theoven 26 and which carries the dummy gage resistance element 84constituting the resistance R; exterior to the oven 26. The leadoutwires composed of the conductors 86 and 88 are balanced by the equallength of compensating lead Wire 90; both the conductors 86 and 38,extending ultimately to opposite ends of the gage 48, and thecompensating lead 90 are attached along a portion of their length to thetubular members 74 of the clamping assembly 3, and are so disposed thatequal lengths of the lead out wire formed of conductors 86 and 88 andthe compensating lead 90 extend within the heated oven 26 during thegage adjustment procedure. This system thus provides that an equallength of wire is in the high temperature area for each of the bridgearms exterior to the indicator 82, i.e. that containing the gage 48constituting the resistance element R and the dummy or compensating gage84 constituting the resistance R thus cancelling the temperature effectson the lead wires 86 and 88. Each of the latter wires 86 and 88 as wellas the arms of the compensating lead 90 are of course duly insulatedfrom each other. The conductor 88 is connected to the compensatingconductor 90 through the short lead 92 in the SR-4 strain indicator 82.The lead wire 86 is connected to the bridge arm 94 containing theresistance R The conductor 90 including the dummy gage 84, constitutingthe resistance R is in turn connected to the arm 96 containing theresistor R The two arms 94 and 96 are connected at the terminal $8, atwhich point the highly sensitive meter branch intercepts the tworesistances. The voltmetercontaining branch 109 is connected to the testgage 48 arm constituting the resistance R and the dummy gage 84 armconstituting the resistance R at the terminal? 101 mid-way along theshort lead wire 92 connecting the two resistances. The alternatingcurrent power source 102 is connected to the bridge within the strainindicator 82 at the terminals 104 and 106 through the lead wires 108 and110, respectively.

The bridge terminals 104 and 106 represent the power or inputconnections for energizing the bridge circuit. The SR-4 strain indicatoremployed is thus seen in the simplified schematic presentation of FIGURE1 as a conventional Wheatstone bridge wherein the temperaturesensitivity of the gage 48 constituting the resistance R is measured asit is being adjusted in the oven 26; thus permiting the gage adjustmentprocess to be curtailed when the desired sensitivity is impartedthereto. The resistances R and R of the arms 94 and 96 respectively areconventional ratio arms and R is the compensating or dummy gage 84 whoseresistance value is known precisely. Resistance values in the bridge aresuch that the changes in resistance induced in the gage 48 by theelevated temperatures of the oven 26 are reflected in an imbalance inthe bridge and a resultant flow of current across the meter branch 100,which provides the measuresent of unmounted sensitivity in the gage 48.It should be noted that the resistance values in the system 4 are notcritical except that they must be matched and must be of sulficientresistance and wattage to prevent excessive current 1Q drain and damageto the resistors. The SR4 strain indicator is a well-known device widelyemployed heretofore for the measurement of static strain. This indicatorhas not, however, been utilized heretofore for the purpose ofcalibrating with accuracy the unmounted sensitivity of weldable straingages of the type described herein.

It should also be noted that suitable observation of the resistancechange in the gage filament due to shifting thereof by the procedureherein described can be had by inclusion in the system of the presentinvention of a Standard Brown Electronic Potentiometer Amplifier(manufactured by the Minneapolis-Honeywell Corporation) and StandardBrown Electronic two-phase motor manufactured by the same corporation orsimilar devices as described in the copending application of theapplicant herein filed on even date herewith and entitled Improvementsin Strain Gages and Methods for the Production Thereof.

The basic embodiment of the strain gage 48 employed in the practi ce'ofthe present invention is illustrated in FIGURES 2 and 3. Gages of thisclass are unique in that they are .Weldably attachable to the testspecimen along the entire effective length of the strain-responsivefilament contained Within the gage. As illustrated in FIGURE 2, the gagecomprises a fine strain-responsive resistance wire or filament 112coaxially disposed in an elongated tube or similar external housing 114with the ends of the filament wire terminating within the tube. A pairof larger lead wires 116 and 118 are welded, soldered or otherwiseattached to the ends of the aforesaid wire filament 112. Alternatively,the lead wires 116 and 118 may the formed integral with wire 112, in amanner hereinafter described, and extend coaxially beyond the respectiveends of housing 114. The free ends of the lead wires are adapted to beconnected to the aforesaid bridge circuit for measurement. A suitablecompactible or compressible mass of solid insulating material 129, suchas mica, aluminum oxide, thorium oxide, magnesium oxide, magnesiumsilicate, forsterite (ZMgO or any of the insulating plastics which aresubstantially stable at relatively high temperatures, such, for example,as polymerized tetra-fluoro-ethylene, is disposed within the housing 114surrounding the fine wire or filament 112. A desirable arrangement forpositioning of the filament 112 within the gage is seen in FIGURE 4. Ithas been found that magnesium silicate ground to a fine powder and firedat approximately 1900 F. for one hour provides an excellent material foruse in the gages of the invention. Two or more insulating materials ofdifferent coefiicients of expansion may be combined in properproportions to give a desired coefiicient of expansion approximatingthat of the material forming external housing 114. A plurality ofinsulating washers 122 may be disposed over the lead wires 116 and 118at each end of the housing to retain the insulating material Within thehousing.

After the insulating material has been placed around the filamentsection 112 and around at least a portion of the lead wire extensions116 and 118 as tightly as possible and the insulating washers 122 areplaced in position to the open end of the external housing 114, thislatter housing 114 is drawn from its original diameter to a smallerdiameter in order to compress the insulating material 120, therebyexerting mechanical pressure on the aforesaid washers 122, theinsulating material 128 and the resistance wire or filament 112 embeddedtherein. This results in the resistance element 112 being clamped overits entire length and surface by the radial pressure of the compressedinsulating material surrounding it. The insulation is in turn firmlycoupled to the inside surface of the external housing 114 by thecompressive force exerted against the housing 114 by the insulatingmaterial 120. As a direct consequence of this arrange ment any movementof the aforesaid housing 114 is transmitted through the compacted massof insulation 120 to the resistance element 112. At the same time theresistance element 112 is thermally and electrically in sulated from thehousing v114i. Thus, if the external housing 114 is Welded to testspecimen 124 by small spotwelds (designated by Xs in FIGURE 2) or by acontinuous weld the entire length of the gage shell in the preferredmanner or any shorter length corresponding to the total length of thefilament 112 contained therein, and a strain is then introduced into thetest specimen 124 causing a modification in its dimensions, thedimensions of the housing 114 will undergo a corresponding modificationor deformation Which will be transmitted through the compressedinsulation 120 to the resistance filament 112 affecting a proportionatechange in the resistance of the latter element 112.

A preferred commercial embodiment of the. gage for use in the presentinvention involves a modification of the basic gage housing 114, asshown in cross-section in FIGURE 3. This modification comprises simplythe addition to the housing 114 of the gage shown in FIG- URES 2 and 3,during manufacture, of a thin, flat strip 121 welded to the tubularhousing'114 at the bottom thereof by small spot welds or by continuousweld the entire length of the filament 112 or housing \114 or anyshorter length corresponding substantially with the effective length ofthe filament 134, and so disposed as to define an extension beyond thesides thereof to provide flat welding surfaces permitting attachment ofthe gage to a test specimen without the use of special electrodes.

Of course, a great many other variations in the basic design of thestrain gage housing 114 may be effected to facilitate weldable mountingof the gage 48-for general use or for specific applications involvingtest specimens of irregular surface contours and the like. Thus, flangesor weldable mounting surfaces of the gage housing 114 may be perforatedor slotted during manufacture to permit riveting of the gage to a testspecimen for improved bonding when mounted by welding or soldering. Thehousing may also be formed of a single piece of sheet steel or othersuitable material by folding to form one flange and a central housingand then welding the free ends together to form the other flange. Afurther alternative permits constructing the housing 114 of a singlepiece with but one flange suitable for attachment by welding to a testspecimen. Too, the central section of the housing containing theresistance wire and insulating material may be pressed into any desiredconfiguration or creased or crimped at one or more points to provideincreased compressive contact following assembly of the gage 48.

An illustration of a further modification of a gage housing which isintended to facilitate mounting thereof on test specimens is seen inFIGURE 5, and comprises an elongated tubular envelope 126, the upperportion of which is substantially circular. The lower portion of thehousing 126 is provided with integrally formed flanges 126a and 12612 onboth sides of the main circular housing 126 extending transversely tothe axis. of the tube. In this arrangement, the resistance element 112,insulating material 120 and insulating retainer Washers (not shown) aredisposed in the housing as in the embodiment of FIGURE 2 and the housingisthen crimped' or run through a suitabie die which gives it thecross-section illustrated in FIG- URE 5, and also causes the insulatingmaterial 1Zti'to be compressed against the resistance element in thesamemanner as efifected by the drawing operation described in connectionwith the embodiment of FIGURES 2 and 3. The flanges 126a and 12612 areintended to facilitate attachment ofthe gage to a test specimen bywelding. Thus, the two flanges are welded directly to the surface of thetest specimen by suitable spaced spot-welds or sin gle continuous linearwelds extending along the entire effective length of the gage, i.e. thelength of the resistance filament 112 plus any excess thereof deemednecessary or desirable. In a similar manner, FIGURE 6 illustrates amodified cross-sectional configuration wherein two separate sections 123and 130, each provided with laterally projecting flanges, are weldedtogether or otherwise fastened at the flanged portions so that theresistance element and insulating material are tightly compacted Withinthe housing. Here again, the gage structure can be mounted on a testspecimen by welding directly through the flange portions.

FIGURES 7 to 9 inclusive illustrate an embodiment of the strain gage foruse herein in which the external housing 132 comprises a rectangularshaped body 134 having an elongated rectangular groove 136 in itssurface. A resistance element 138 comprising two relatively heavyelectric leads 140, 142 with a fine gage wire 144 bonded between thelead wires is disposed within the groove. Each of the lead wires restson sheets 146, 148 of suitable insulating material, such as mica.Additional sheets 1-50, 152 ofinsulating material are placed on top ofeach of the lead wires. A pair of small rectangular blocks 154, 156 aredisposed over the stacks ofsheet mica, respectively. These blocks areforced down on the mica sheets by any suitable means such as hammeringand are held in position by deformation of the upper edge of the grooveadjacent the upper surface of the blocks, as indicated at 15??? by thedotted lines in FIG. 8. A powdered insulating material 160, such asmica, is placed in the groove between the two sheet stacks and tampe'dfirmly into the groove.- A bar 162 adapted to fit in the groove isdisposed over the powdered mica and is hammered and clamped in positionin the manner described for the two small blocks. The dotted lines at164 of FIG. 9 illustrate how the. bar is held in position. With thisarrangement the fine Wire is firmly clamped in the powdered mica so thatthe wire resistance will respond to deformation of the body,

In the embodiment shown in FIGS. 7 through 9 the body can either be thetest specimen proper or it can form a suitable housing which is adaptedto be bonded to, a test specimen. In general, this type of structurefinds particular application in permanent installations, wherein acontinuous strain measurement or similar physical measurement isrequired during routine operation of the equipment forming the testspecimen, such as for cranes, lifts, and the like.

As pointed out hereinbefore, the housings of the strain gages employedin the practice of the present invention are preferably formed of amaterial which is readily adaptable to bonding by electric resistancewelding of sonic welding. On the other hand, it is equally important "tosuccessful operation of the gages that the housing be of minimumstructural strength so as to reduce its effect on the normal straincharacteristics of the test specimen, and to permit it to be deformedreadily under stresses occurring within the test specimen. It has beenfound that housings formed of thin sheet steel are admirably suited foruse in the manufacture of weldable strain gages according to theinvention and that such housings may be formed in a variety ofconfigurations to facilitate attachmerit of the gages to a test specimenby welding while at the same time permitting the housings to be formedof minimum structural strength. In adapting the basic structuralcharacteristics of these gages to conventional welding practices, it isimportant that the gage housing be such that it will permit positioningof the welding electrodes as close as possible to the actual point orpoints of weld, thereby to insure the shortest possible path for thewelding current and substantially localized heating only, of the gagestructure.

For example, with reference to FIG. 10 of the drawings, there is shownin cross-section, the basic gag-e configuration of FIG. 3 positioned forattachment by welding to a test specimen 166. Current from the weldingelectrode 163 will flow within the tubular housing 11 as indicated bythe small arrows in FIG, 10, entering the test specimen at theweld-point, as indicated by reference numeral 170. Although it has beenfound from practical experience that it is entirely possible tospot-weld a gage of plain tubular configuration in the mannerillustrated, and to obtain accurate test results with gages mounted inthis manner, it has also been found that since the welding electrode 163must be positioned relatively remote from weld-point 176 in this type ofwelding operation, unless one employs a special electrode that isadapted to fit around the tubular housing of the gage there is somedanger of damaging the gage through overheating of the tubular walls.Furthermore, it is difficult in this type of welding operation to obtaina high concentration of force at the welding point because of thepossibility of crushing the relatively thin tubular housing under thepressure required.

As explained hereinbefore, in the gage structures illustrated in FIGURES3, 5, and 6, provision is made for attachment of the gage by welding orspot-welding to a test specimen by means of the strip 121 in FIGURE 3 orflanges 126a and 126b formed integrally on each side of the lowerportion of the housing 126 as seen in FIGURES 5 and 6. With this type ofgage housing, the welding electrode can be positioned directly over therespective flanges without excessive heating of the remaining portionsof the gage housing and without danger of crushing the central sectionof the gage housing containing the resistance element. It sometimesoccurs with this type of structure (FIG. 5), however, that in the dieoperation or crimping operation employed to compress the insulationabout the gage wire or wires and for forming flanges 126a and 126b, aportion of the insulation may be compressed between the flange walls,thereby forming a relatively high resistance path for welding currentpassing through the flanges to the-test specimen.

For-certain applications, it is desirable to terminate the gagelead-in-wires at one end of the gage housing and in FIGS. 11 and 12,there; are shown in sectional-plan and cross-sectional views,respectively, a gage of this type.

With reference to FIGS. 11 and 12, the gage comprises a substantiallyflat bottom section 172 and a semi-tubular upper section 174 definingsubstantially the entire housing for the resistance wire 112 andinsulating material 120. The uppersection 174 is closed on three sidesand is provided with flange section 176 which are welded or otherwiseattached on'three'sides to the bottom sect-ion 172. The gage is formedin the manner described in copending application Serial No. 754,956Improvements infStrain Gagesand Installation of the Same, filed August14, 1958,'by the applicant herein. The resistance wire 112 is loopedWithin the housing as shown in FIG. 11 and is joined to the separatelead-in wires 116 and 118 within the" housing, and these leads are thenextended externally of the housing at the single open end throughsuitable insulating washers or retainers 122. The upper section 174 ofthe ga'gehousing may be dished to a greater depth over the portionhousing the lead-in wires, as indicated by reference numeral 178 in FIG.11, to accommodate these wires.

In FIG. 13, there is shown a form of gage housing which is ideallysuited for use for strain measurement work performed under conditions ofextreme moisture or under corrosive conditions, as, for example, thoseencountered in underwater measurements or measurements 7 Within concretestructures wherein the gage must be positioned for use before the wetconcrete is cast or permitted to set. With reference to FIG. 13, thereis shown one end only of a gage similar to the basic gage of FIG. 2,where- I in the insulating retainer washers 122 are replaced with ahousing 114, or, alternatively the seal can be formed after the gage hasbeen deformed to compress the insulation. The housing can be evacuatedbefore sealing or an inert gas inserted under pressure it necessary ordesirable.

The hermetical seal eflectively solves the more troublesome problems ofmoisture and corrosion and renders the gage, with proper long lead wiresof the covered type hereinbefore described, usable under water withcomplete safety and usable in concrete where moisture conditions arealso a serious problem.

In adapting the basic gage for use in strain measurements withinconcrete structures, it is preferred to provide external housing 114with a plurality of fins or projections, as indicated by referencenumeral 114a in FIG. 13 to interlock with the concrete thereby insuringa better bond between the gage and structure under analysis. Any similarirregular surface provide-d on housing 114- will accomplish the desiredresults.

In lieu of the compressible powdered insulation, or in addition thereto,it has been found possible to form insulated filaments by simplyenamel-coating the otherwise bare wire, or by coating the same with alayer of oxide deposited on the wire cataphoretically or by theconventional drag or spray methods used by vacuum tube manufacturers incoating heater elements for use in vacuum tubes. The insulated wire isthen placed in any suitable external metal housing such as thosedescribed earlier, with or without additional insulation of the powderedtype. The use of a coated wire 112a (FIGS. 14-15) facilitates handling,reduces assembly costs, provides a gage of better quality and simplifiesthe fabrication of more complicated gages. It is also possible toelectrocoat or deposit a metal shell over an oxide-coated filament ofthe foregoing type to provide a complete gage structure for direct useas such, or for mounting within the normal external housing as apre-assembly expedient.

Deposition of a metal shell over the filament-insulation unit isaccomplished in the same manner as deposition of the insulation, or thedeposited insulation can be precoated with graphite and the metal shellthen electroplated over the graphite. Alternatively, the graphite-coatedassembly can be set up as the dissociation-deposition surface in astandard van Arkel-de Boer bulb with the metal coating eposited bydissociation of a metal halide in cont-act with the assembly whilemaintained at an elevated temperature by the passage of electrodecurrent through the filament.

FIGS. 14 and 15 illustrate another embodiment of the invention in whichan insulation-coated filament wire 112a can be used advantageously. Theprecoated element 112a is laid lengthwise on top of a rectangular pieceof thin metal foil 182. with uncoated ends 184-185 of the filamentextending beyond the foil. The element 112a is provided with two smallbeads 1881% of insulation which are positioned along the wire so thatthey are adjacent the ends of the foil 182. A piece of metal foil 192,similar in size and shape to the lower foil 132 is placed on top of theelement 134a and in registry with foil 182. The two foils 132 and 1% arejoined together by welding along each side of the wire 112a. Thus, asplit shell tube type of construction is formed with the insulationfirmly clamped to the wire element and the external housing.

A still further embodiment of a wire strain gage for use herein in whicha precoated wire may be used to advantage is shown in FIGS. 16 through19. The gage wire 112 formed as one piece with large end portions orleads 194 196 is first coated with insulation by any of the aformentioned methods; the insulation extending over the thin portion of thewire and partially onto the enlarged end portions as shown in FIG. 17. Alayer of metal 198, such as nickel, is deposited over the insulationexcept for a small amount of the insulation at each end as shown in FIG.18. The precoated element is then placed in a metal tube 2% which isslightly shorter than the length of the metai layer 198. The centralportion of the tube 200 is then swaged so that it is made smaller thanits original diameter. This serves to powder the insulation in the.central portion and so frictionally couples the wire 112; the insulation120, the metal layer 198 andthe tube 200 into a unit strain-responsiveassembly. The tube 200 is then provided with an elongated metal flangesecured to it, as by welding, which serves as a means for securing thegage to a test specimen (FIG. 20).

Significant advantages of the frictionally-coupled gages thus describedare their weldability, which provides for ease in mounting on a testspecimen, and the fact that they may be employed equally well in hightemperatures or ordinary measurement work without fear of affectingthepressure coupling between filament: and external housmg.

As shown, for example, in FIGS. 2 and 11, the resistance elements of thegages employed herein consist of a fine Wire bonded, as by soldering orwelding, between two larger electrical lead wires. The fine resistanceelement can also. be formed from a single, larger wire, such, forexample, as wire 204 shown in FIG. 4 of the drawings, by drawing the"wire down to a smaller diameter intermediate its ends'to increase theresistance of the central portion thereof which may then be arrangedwithin housing 114 in the form of a grid by doubling it back in severalloops to' provide increased sensitivity or greater resistancewlierenecessary.

It hasbeen found, however, that the production of an internal-resistanceelement or filament for these gages by soldering or welding of separatelead wires to the fine wire 112, is a precise and tedious operationowing to the very small sized wires employed (0.001 inch and less).Furthermore, the resulting joints are a possible hindrance totrouble-free operation of the gages because of possible thermocoupleaction, failure, etc. On the other hand, in forming the filament from asingle wire by drawing in the manner described above, it is difficult tocontrol or regulate the ohmic resistance per unit length ofwire withoutthe exercise of precise measuring techniques.

Accordingly, it has been found that a filament-lead wire unit of thegeneral type illustrated in FIG. 4, in which the strain sensitive wireand lead wires are formed integrally, can be produced by electroplatinga length of fine Wire selected for the resistance element, to build upthe portions on either side of the filament section thereby providingintegral, enlarged lead wires. On an individual production basis, awireselected to provide-the desired-diameter for the'resistance element, canbe masked by coating or by the use of a clamp or tube to block off acentral portion of proper length for the resistance filament and theentire wire may then be placed in the plating solution until theunmasked end sections are built up to the desired leadwire diameter byelectroplating. Although the plating procedure is preferred,substantially the same result can be accomplished by etching away alarger wire to provide a reduced diameter filament section.

There is shown in FIGURE 21 an arrangement whereby these unitaryfilament-lead type elements can be mass produced by electroplating. Inthe drawing there is illustrated an electro-plating jig or frame ofrectangular form which is adapted to be placed in a suitable tank (notshown) containing an electroplating solution. The jig comprises a topcross-member 206 of metal such as bronze, and two side supports 208 anda bottom crossmember 216, all formed of insulating material, such asMicarta. A plurality of metallic pins 23 .2 are provided in spacedrelationship across top member and similar pins 214 of insulatingmaterial are provided across bottom member 210. The line resistance wire215 to be used as the gage filament is wound tightly between pins 212and 214 in looped fashion as shown in HS. 21 and the ends anchored tothe jig in any suitable manner. The jig is then placed in anelectroplating bath to a controlled epth, as represented by the dottedhorizontal line 218 in FIG. 21, The electroplating apparatus includes acon- 16 ventional anode formed of the metal to be deposited connected tothe plus side of a source of potential, the negative side of whichcan beconnectedv tov metallic cross member 2% of the electroplating jig.

Following the plating cycle, the jig is removed from the bath and thepartially plated, partially unplated wire is removedfrom the jig and thelower platedloops are cut as-indicated by dotted line 220 in FIG. 21.The resulting segments comprise a plurality of separate filament wireswith plated lead-wires at each enclthereof. The resulting [relativelylarge lead-wire sect-ions can be shortened, twisted, bent, etc., withoutfear of failure such as occurs in joined wires, and;without affectingthe basic characteristics of the resistancev element since'theirresistance is quite small compared. to that of the. filament section. Itshould be readily apparent that the electroplated filamenh lead Wireassembly is usable in a great variety of other electrical devicesbesides strain gages. Thus, many electrical devices such as wire. woundresistors, relay coils, etc., utilize a length of relatively smallwirewhich forms the key electrical circuit of the device... In almost.all items of this'type, eachend of the fine: wire. mustbe equipped'witha short section of larger wireor IQadQOth nector and this is customarilydone by brar.ing,.welding or physically clamping the required large wireto the-fine wire; Besides. providing the ultimate in strength. andelectroplated with approximately a- 0.002- inch-"layer of nickel ina,Watts Bath type of solution of are following composition:

Hydrogen peroxide 1 pt./ l0.00 1gal.

Nickel sulfate 40 oz./ gal. Nickel chloride 6 oz./ gal. Boric acid 4.oz./gal.'

No brightening or hardening agents were included as these tend to impartbrittleness. The temperature otthe bath was maintained at 150 F.l60 andthe pH (Electro) at 274. The current density was over: A.S.F. The partswere cleaned prior to electroplating according to standard procedures,i.e., dipped in alkaline solution, rinsed in water, dipped. inacidsolution and rinsed. in water again. The plating cycle is completedin approximately 8-l5 minutes. The resulting product consisted. of anunplated (0.001 in. dis.) filament sedtioni approximately'one-inch inlength, and two leadwireof 0.005 inch diameter, each approximatelyone'inch in length.

A more elaborate method and apparatus for producing the unitary typelead wire-filament assembliesfhas: been described and claimed incopending application Serial No. 669,144 which was filed on July 1, 1957by the applicant herein.

In a typical installation of the gages. o f-the. invention, inparticular, a gage of the typeillustrated in FIGS. ll and 12, there isemployed a condenserdischarge type of welder. Spot weld beads are formedbetweenthe flan es dependability compared with a welded or othe ,i sejointed assembly, the electroplated assembly offers a tremendousreduction in manufacturing. costs,

During the. plating cycle the level of-the electroplating solution canbe automatically regulatedby a simple float valve to keep it at aconstant level or it maybe varied-to effect a gradually receding levelthereby to produce tapered sections such as that illustratedby lcadrend204a of the filament illustrated in PEG. 4. By'tap e ring-the leadsection of thewire in this manner, the holding'force of the filament isincreased whenit isclamped-in the external housing by the compressed,insulating material. Alternatively, following the initial plating cycleto build up a suitable lead-wire diameter, by folding the lower platedloops of the wire upwards out of the. bath but leaving a small segment.of the plated. leadsection adjacent the filament section within thesolution, a short oversized anchoring section or lug can be plated,which also. serves to increase the holding force. of the compressedinsulating material against the filament when the gage is assembled. Ofcourse, the surface of the filament section as well as those portions ofthe lead wires contained within the gage housing can be roughened orcoated with abrasive particles and the inside wall of the housing may besimilarly treated to increase the frictional holding force of theinsulating material within the assembled gage.

In a typical example of the foregoing electroplating technique, a 0.001inch diameter Evenohm (trade nameapproximately 80% nickel and 20%chrome) resistance wire was of the gage and the test specimen along bothsides of the active section (filament) of the gage. The two rows ofbeads are formed as closely as possible to the center portion of thegage housing by shaping a No. 3 welding electrode to provide an end ofapproximately 0.020 X 0.050 inch, the long dimension of the electrodebeing located parallel to the axis of the gage. The weld beads may beformed approximately 0.030 inch apart along the entire active length ofthe gage. Of course, special welding electrodes such as a wheel or diskwhich rolls along the flange or both flanges of the gage and is adaptedto fire the welding current according to its linear and/or rotarytravel, through a commutation system, may be employed to facilitate themounting operation. Electrodes of this type are described and claimed inapplicants copending applications Serial Nos. 721,255 and 721,256 whichwere filed on March 13, 1958.

It is difficult to accurately depict the gage structures of theinvention simply by reference to the drawings because of their extremesmallness, and, accordingly, it is believed that a dimensional breakdownwill facilitate a complete understanding of the invention. Roughly, thegages measure up to two (2) inches in length, the outside diameter ofthe gage housing being approximately 0.020 inch, and the overall widthof the gage housing including attachment flanges measures approximatelyone-eighth of an inch. The following additional dimensional data willfurther facilitate an understanding of the invention:

Fine resistance wire 0.005 to 0.002 inch.

Lead wires 0.003 to 0.010 inch.

Housing diameter (O.D.) 0.015 to 0.020 inch (up to 0.050 inch where leadwires ends enlarged).

Housing thickness 0.001 to 0.002 inch. Flange width 0.020 to 0.030 inch.Flange thickness 0.002 to 0.004 inch. Length of gage A to 2 inches(extendable The high temperature type gages of the invention have beenoperated with satisfactory performance at temperatures within the range1800 to 2000 F., thus providing a temperature stability not attainableheretofore with conventional bonded or unbonded strain gages, whileproviding gages capable of measuring compressive stresses, as well asthose due to tension.

It is desirable for certain specific applications to provide a pluralityof resistance elements or filaments within a single gage housing.Illustratively, the simplest form of multi-filament gage is one whereintwo separate resistance wires are carried within the same housing inparallel relationship and simply insulated from each other bymaintaining a spaced relation therebetween within a mass of insulatingmaterial. Another example is one which includes a complete four elementbridge circuit contained within a single housing. Both modifications aswell as others prepared and described in copending application SerialNo. 754,956 filed by the applicant herein on August 14, 1958, andreferred to hereinabove, may be temperature compensated by the generalmethod of the present invention.

For other measuring techniques, it is more desirable to use a pluralityof independent gages which may be assembled in clusters of apredetermined configuration, i.e. rosettes, shear gages, complete fourgage bridges, and the like with desired angles and spacings betweenrespective gages.

The strain gages adjusted and calibrated in the manner described hereinmay also be used in combination with conventional fittings for measuringa variety of physical quantities. That is to say, strain results fromstress which in turn result from a force produced by pressure,acceleration, etc. Thus, a device sensitive to strain may be used tomeasure stress, force, pressure, acceleration, displacement and torque,among others. For example, a torque meter can be formed simply byequipping a shaft with four weldable gages. The gages can be installedin the same manner on rings, links, beams, tubes, etc. to providesensing devices capable of measuring most physical quantities. Inaddition, the gages may be used in conjunction with known refinementsrelating to conventional gages such as high sensitivity resistanceelements, etc.

Apart from the many advantages of these gages as have been pointed outin the foregoing description, it is also important to mention that theresistance to ground (between resistance element and the test specimen)is unaffected by installation and therefore can be specified andguaranteed to the consumer. The insulation characteristics of aconventional bonded gage cannot'be determined prior to installationsince it is affected by the installation. Actually, in some instanceswhere a bonded gage is installed on a rough surface, the gage insulation(paper and bonding cement) gets punctured and a short to ground results.This cannot occur with a weldable gage since the insulation is disposedbetween its own housing and the fine resistance element. The housing isdirectly and deliberately grounded to the test specimen by welding butthe'insulation surrounding the resistance wire remains undisturbed. Thatis to say, the installation procedure is conductive in nature ratherthan insulating, and cannot afiect the resistance to groundcharacteristics of the unmounted gage. Furthermore, the metallic housingprovides electrical shielding for the resistance element as well asprotection against damage through handling or misuse. In the actualtests conducted with typical gage structures of the invention, it isfurther found that they are not affected by radiation phenomena as isthe case with known forms of bonded gages.

An additional and significant feature of the strain gages of the presentinvention is their long-term temperature stability. Since temperaturecompensation and long-term stability against drift are not alwaysdirectly related, however, it is desirable in certain instances toforego the ultimate in sensitivity adjustment or temperaturecompensation to affect enhancement in extended stability of the gage;thus inhibiting excessive drift therein when maintained at a constanttemperature for an extended period of time, e.g. several weeks. It hasbeen found, however, that curing of the gage in a suitable manner evenat the desired adjustment temperature tends to impart a substantiallong-term stability thereto. Thus, gages in which the filament is formedof Evenohrn wire, of'0.005 to 0.001 in. thickness, undergoing a normalcur ing cycle of approximately ten minutes duration at an oventemperature in the range of 800 F. to 900 F. and preferably ofapproximately 850 F. for downward adjustment of temperature sensitivitypossess a substantial stability over extended periods up to 700 F.

The temperature range at which downward adjustment occurs is ratherlimited, i.e. 800 F. to 900 F. The procedure is, in addition, relativelyslow. However, long term stability to drift at temperatures in excess of700 F., e.g. up to 900 F., is imparted to the gages of the presentinvention most desirably at the aforesaid adjustment oven temperature of850 F. by simple prolongation of the curing cycle, i.e. approximatelyone to two hours. The upward adjustment in temperature occurs at oventemperatures in the range of 900 F. to 1500 F. and preferably from 1100F. to 1200 F. Significant, but lessened long-term stability to drift isattained by a similar prolongation of the curing period at oventemperatures in excess of 1500 F., i.e. up to approximately 2000 F. Itwill be evident that the resistance element or elements as of the oven26 must be capable of imparting to the oven 26 temperatures within theseaforesaid ranges, that is temperatures sufiiciently high to produce thedesired shift in bridge balance resistance and temperature sensitivity,and sufficiently low so that this shift is gradual enough to permitobservation and shut-off at a desired value. Further provision forsecuring a long-term stability against drift is had if the finaladjustment temperature compensation is made by lowering the sensitivitythereof to the desired value rather than raising it. This, of course,does not obviate use of the technique wherein the adjustment of the gageis raised to the desired sensitivity value, a procedure which is bothconveniently and rapidly performed, but it suggests that it is better toovershift the adjustment values upward and subsequently make the finaladjustment in a downward shift.

As noted hereinabove, the sensitivity thus imparted to the unmountedgage by these above described procedures is a significant and ameaningful one permitting an accurate measurement thereof before usewhich has been hitherto unknown in strain gage construction. The outershell of the gages of the class described hereinabove is significant inrelation to the instant methods of adjustment and measurement,particularly as regards the temperature coefiicient of expansion of theshell or outer housing. Thus, it will be evident that this housing isnot a major factor when a gage is mounted on a test specimen since beingthe weaker of the two in construction, the gage shell follows theunderlying structure in the same manner as it would bring a change instructure induced by stress. However, when the gage is unmounted, thissame shell or housing is inhibited by the underlying structure of a testspecimen, but provides a significant strength to yield a meaningfulunmounted sensitivity in the gage itself. Thus, in effect, the gageshell or housing replaces the test specimen in the unmounted state andin this manner provides an ideal structure for obtaining accurate andrepeatable unmounted sensitivities. While, in the unmounted state thegage housing resists bending and is strong enough to be a controllingfactor in the overall expansion rate of the gage, rendering as noted, anaccurate unmounted sensi tivity thereto, the relation between unmountedand mounted sensitivity must be known. Thus, for example, where 304stainless steel is employed in the gage housing unmounted and mountedsensitivities should be substantially identical, since the thermalcoefficient of expansion of the gage housing and therefore of theunmounted gage is the same as the test specimen. This being a desirablecondition, it will be evident that the unmounted sensitivity of a gageshould be as nearly Zero as possible where gage shell and test metal arethe same, so that its mounted sensitivity is as low as possible. Where,then, gages are made of the same metal as the test specimen, thiscondition is substantially achieved. However, where gages are made, forexample, of 304 stainless steel and the test specimens on which thesegages are to be mounted are made of dilferent metals, the unmountedsensitivity of the gages will have to be adjusted in a substantiallydirect ratio to the difference in the thermal coetficients of expansionof the gage and test specimen. illustratively, metals having aco-eificient of expansion higher than that of 304 stainless steelrequire a lowering of the unmounted sensitivity, i.e., a

negative unmounted temperature sensitivity; and metals with a lowerthermal coefiicient of expansion require a more positive unmountedsensitivity. To illustrate further, if a gage with an unmountedsensitivity of zero were mounted on aluminum, the gage would bestretched more in the mounted position with increasing temperaturebecause of the higher rate of expansion of aluminum than that of thegage housing made, again illustratively, of 304 stainless steel. Thus,it would have a high positive sensitivity when welded to aluminum. Toobviate. this high positive sensitivity, a proper gage to be used on thealuminum should have a compensation of higher negative unmountedsensitivity. Specifically, if a gage for a 304 stainless steel housingis to be used on a test specimen constructed of 1500 aluminum, theunmounted sensitivity of the gage should be approximately 1500micro-inches/in./ F. for a temperature change from F. to 600 F, adifference of 500 F., or an average sensitivity of 3 micro-inches/in./F.

When the gage is tobe used on 4130 steel the unmounted sensitivityshould be approximately 1+1000 micro-in./in./ F. for the same 500 F.temperature change, or an average of +2 micro-in./in./ F. These figuresindicate a difference in the average thermal coelficients of expansionof aluminum and 4130 steel over this range of 5 parts per million F. (adifference between 3 and +2 micro-in./in./ P.) which is in substantialagreement with known figures for these coefficients in these temperatureranges.

Since there is a relationship between coefiicient of expansion andunmounted sensitivity, this coefiicient, if known, can be used topredict the unmounted sensitivity required. In addition, the gage can beused to measure coefficient of expansion. A zero sensitivity in themounted gage can be attached to the material in question. The mountedsensitivity in micro-in./in./ F. is then a good indication of thediiference in expansion coefficients between the 304 stainless shell ofthe gage and the material in question. If the thermal coefficient ofexpansion of the 304 is known then the coeflicient of expansion of thematerial to which it is attached is readily computed. Of course, thecoefficient for 304 stainless steel can be accurately determined bymounting gages on materials with known thermal coemcients of expansion.

The eifectiveness of the compensation may be checked by mounting ga eswith known unmounted. sensitivities on a sample piece of the material tobe tested and plotting apparent strain against temperature. Two or threechecks may be made before the proper unmounted sensitivity is arrivedat. For example, if, on the first attempt, a gage with an unmountedsensitivity of +900 m'icro-in./in./ F., for a change of 500 F., ismounted and found to ave a sensitivity of 200 micro-in./in./ F. for thesame temperature change; then, on the next attempt a gage with anunmounted sensitivity of +1100 micr0- in./in./ 500 F. may be tried, iean additional. sensitivity of +200 micro-in./in./ F. tocounteract theprevious -200 m-ioro-in./-in./ F. in the first mounted gage. Thefollowing is a table showing approximate values of unmountedsensitivities for some common metals:

TABLE 1 Micro-inJinJ Miero-in./in./

The approximate thermal coefficients of expansions of these metals arelisted below:

304 Stainless Steel Titanium- 4130 Steel 304 Stainless AluminumMagnesium-Thorium It can be seen that, if 304 stainless is employed inthe gage shell or housing the unmounted sensitivities are inapproximately inverse ratio to the thermal coetficints of expansion.

It should be noted that some alloys possess a wide Variety of thermalcoeflicients of expansion depending on the heat treatment. Thus, anerror of over 1000 mioro-in./in./ F. for a 500 F. change can beintroduced by this factor so that there is no substitute for using asample of material which is identical to the test structure, not only incomposition but in heat treatment as well, on which to mount andcalibrate gages to arrive at the proper unmounted sensitivity.

It will also be noted, that there is an ignored change in the bridgebalance for a very short period after the gage is subjected to the highoven-heat. This represents the shift in resistance caused by the actualbringing up to temperature of the filament. It is not a permanent changeand will reverse if the temperature is dropped back immediately, i.e.returned to the original room temperature conditions. The permanentchange in resistance which occurs after the gage has attained thedesired temperature in the oven 26 is that which is significant. This issupported by an observation at ambient temperature of the bridge balancebefore and after each curing cycle.

It is noted that the oven temperatures used for upward shifting of gagesensitivity can also be employed in offecting a downward shitt in gagesensitivity. Thus, in increasing the oven temperature to efifect anupward shift in sensitivity the gage filament must pass through thelower temperature range referred to above, e.g. 800 F. to 900 F., and ifthis transition is made sufiiciently gradually, the increase inresistance will be effected in this lower range. Subsequently, as thetemperature gradient is increased in the higher ranges, a reversal insensitivity will occur. It is also noted that the weldable strain gageemployed herein can be used for temperature measurement by adjustment oftemperature sensitivity to a known high value rat-her than to a lowvalue.

Thus, it will be evident that there is provided by the presentinvention, a temperature compensated weldable wire resistance straingage accurately adjusted and measured in its unmounted state, as well asnovel and unique methods for the production thereof.

What is claimed is:

1. An unmounted strain gage temperature compensated for a particulartest specimen to which it is to be attached by welding comprising:

the combination of a strain responsive resistance filament coupledthrough compactible electrical insulating material to a metallic housingalong its entire effective length;

said combination having a resistance change versus temperature of avalue which will cancel the difference in the effects of thecoe'fiicients of expansion of the gage and the test specimen.

2. A strain gage as claimed in claim 1, wherein said metallic housingcomprises a tubular housing surrounding said filament and insulation andwhich compresses said insulation against said filament, and a thin, fiatmetallic strip welded to said housing along the entire length thereof.

3. An unmounted strain gage temperature compensated for a particulartest specimen to which it is to be attached by welding comprising:

the combination of a strain responsive resistance filament coupledthrough compactible electrical insulating material to a metallic housingalong its entire effective length;

said metallic housing being of the same material as said test specimen;

said combination having a resistance versus temperature sensitivity ofapproximately zero.

References Cited by the Examiner UNITED STATES PATENTS 1,127,373 2/1915Read 219-540 1,127,374 2/1915 Read 219540 1,359,400 11/ 1920 Lightfoot338238 1,973,629 9/1934 Hofer 29-15564 2,271,975 2/1942 Hall 29--1'55.632,36 ,181 11/1944 Howl-and 338 -2 2,543,708 2/1951 Rice et al. 266-52,556,962 6/1951 Field 2665 2,639,246 5/1953 Dunlap 148-43 2,725,31711/1955 Kleimack 14813 2,812,409 -1-1/1-957 Jones et al 338-2 2,913,690'11/ 1959 McGrath 3382 2,935,709 5/ 1960 Paine 3382 RICHARD M. WOOD,Primary Examiner.

MARCUS U. LYONS, NATHANIEL MARVEL- STEIN, ANTHONY BARTIS, Examiners.

H. T. POWELL, W. D. BROOKS Assistant Examiners.

1. AN UNMOUNTED STRAIN GAGE TEMPERATURE COMPENSATED FOR A PARTICULARTEST SPECIMEN TO WHICH IT IS TO BE ATTACHED BY WELDING COMPRISING: THECOMBINATION OF A STRAIN RESPONSIVE RESISTANCE FILAMENT COUPLED THROUGHCOMPACTIBLE ELECTRICAL INSULATING MATERIAL TO A METALLIC HOUSING ALONGITS ENTIRE EFFECTIVE LENGTH; SAID COMBINATION HAVING A RESISTANCE CHANGEVERSUS TEMPERATURE OF A VALUE WHICH WILL CANCEL THE DIFFERENCE IN THEEFFECTS OF THE COEFFICIENTS OF EXPANSION OF THE GAGE AND THE TESTSPECIMEN.